Composite materials are known to have the advantages of high strength, light weight, design flexibility, dimensional stability, corrosion resistance, parts consolidation, better finishing, and low tooling cost over traditional construction materials such as metal, ceramics, and wood. Fiber-reinforced thermosetting polyester composites are widely used in many applications, e.g. marine, automotive, transportation, electrical, construction, consumer and industrial goods, etc. Compared to the composites made from other types of thermosetting resins such as vinyl ester, epoxy, and polyamide, thermosetting polyester composites have the advantages of lower material cost and easy material handling during processing. Therefore, unsaturated polyester resins are the materials of choice for most of the fiber-reinforced thermosetting composites in applications in which the working environment of the composite is not very harsh.
Fiber-reinforced thermosetting polyester composites usually consist of reinforcing (or reinforcement) fibers, either in chopped or continuous form, embedded in a matrix of one or more unsaturated polyester resins. The unsaturated polyester resins are made from a reaction of one or more glycols with an unsaturated dicarboxylic acid or its anhydride, or with a mixture of the unsaturated acid or its anhydride with a saturated dicarboxylic acid or its anhydride. The molecular weight of the polyester is controlled through any conventional means, e.g. by the addition of dicyclopentadiene to the reaction mixture.
In the formation of the matrix, the unsaturated polyester resin is blended typically with (1) one or more monomers capable of crosslinking with the vinyl groups in the polyester, (2) one or more peroxide initiators, (3) promoters for use in combination with the initiator, and (4) various other additives which impart desired characteristics to the matrix upon cure or which will improve the processing and/or curing properties of resin. This precured blend of components is known by various names including matrix precursor, matrix reaction mixture, and the like.
The physical and chemical properties of the composite, such as its physical strength, physical modulus, flexibility, and heat distortion temperature, can be controlled by appropriate selection of the starting glycols and dicarboxylic acids in the manufacture of unsaturated polyester resin, or the crosslinking monomers, initiators, fillers, fibers, and other additives used in the preparation of composite. As a result, a wide array of fiber-reinforced thermosetting polyester composites can be prepared through the appropriate selection of starting materials, and some of these composites are particularly useful in the manufacture of strong, relatively light weight plastic parts.
Various processing methods can be applied to produce fiber-reinforced thermosetting polyester composites. The hand lay-up and spray-up processes are the most common practices in the manufacture of large and complex composite parts, such as boat hulls and truck body panels. Continuous or chopped fiber mats are impregnated with and engulfed in a matrix resin, and the resin is cured without additional heat or pressure. The typical fiber reinforcement (e.g. glass fiber) content of a composite made by these techniques is about 20 to 40% by weight, based on the cured weight of the composite. Therefore, the physical strength (as measured by any one of a number of different tests) of these composites is typically not very great and if greater physical strength is desired for a particular application, then a thicker composite is usually required (the physical strength of a composite being a function of the fiber content of the composite and its thickness). Moreover, the surface appearance of the finished part made with these methods may vary from part to part depending on various factors, e.g. processing conditions, the nature of the thermosetting resin, and the like.
Thermosetting polyester composites with better physical strength and/or consistent surface appearance can be produced by other types of manufacturing techniques, such as filament winding, compression molding, transfer molding, injection molding, and pultrusion. These techniques can produce parts with very high fiber content, typically 50 to 70% by weight. However, the nature of these processes, and in some the added tooling and operational costs, prevent their use in the manufacture of very large and complex parts such as those described above.
With the introduction of vacuum-assisted transfer molding as described in U.S. Pat. No. 4,902,215 and U.S. Pat. No. 5,052,906, both of which are incorporated herein by reference, very large, complex and physically strong composites can be manufactured with relatively low tooling and operational costs. Composites made by this technique lend themselves well to the manufacture of large, complex, strong and relatively light weight plastic parts. However, because a composite made by vacuum-assisted transfer molding has a very high fiber content, the cosmetic surface appearance of the composite is more sensitive to the shrinkage that naturally occurs during the cure of a thermosetting polyester resin. significant fiber pattern print through can be observed, sometimes even through both a skin laminate and gel coat applied to the surface of the composite construction. Correction of this problem by sanding and polishing after the composite is made requires considerable effort which undermines, or even may eliminate, the savings in operating and material costs otherwise gained from using a vacuum-assisted technique.
The composite industry holds a continuing interest in the development of a method for the manufacture of a fiber-reinforced thermosetting polyester composite that possesses both great physical strength, relative to a composite made from a traditional hand lay-up and spray-up method, and a smooth surface appearance relative to a composite made from a vacuum-assisted method. Such a composite will be a ready candidate for molded parts, especially parts of large size and/or complex shape, requiring both physical attributes.